My ti road frame doesn't use a replaceable one. My wife has broken one on her ATB and I've broken two on mine. They were all from sticks that got caught in the chain.
The hangers are sometimes hard to come by, so I keep spares. I carry one in my hydration pack tool kit as i had to walk out pretty far once. If I didn't have them on the ATB's, I might be out rear derailleurs. They cost more but are easier to get. On one occasion, I still had to replace the rear derailleur as the jockey pully cage got bent.
Al